Dew point hygrometer



Jan, 6, 1953 DE WITT T. VAN ALEN 2,624,195

DEW POINT HYGROMETEIR Filed Oct. 25, 1946 3 Sheets-Sheet 1 9 Il l' S W4 l//ns l FIG. 7

l INVENTOR o l' A DEWrrT T. VANALr-:N L; /I Il BY I l7l\/ ll/72 mi M AMOUNT OF DEW ATTY ATTY.

DE WITT T. VAN ALEN 2,624,195

DEW POINT HYGROMETER 3 Sheets-Sheet 2 Jan. 6, 1953 med oct. 25, 1946 Jal-, 6, 1953 DE wlTT T. VAN ALEN 2,624,195

DEW POINT HYGROMETER 3 Sheets-Sheet 5 Filed 001,. 25, 1946' INVENTOR DE W|TT T. VANALEN ATTY Patented Jan. 6, 1953 DEW POINT HYGROMETER De Witt T. Van Alen, Delavan,

The George W. Borg Corporation,

Wis., assignor to Chicago, Ill.,

a corporation of Delaware Application October 25, 1946, Serial No. 705,689

(Cl. 'i3-17) 22 Claims.

The present invention relates in general to dew point hygrometers, and the obj-ect of the invention is a new and improved instrument of this character.

The invention may be considered as an improvement on the invention disclosed in the application of Brissman et al., Ser. No. 584,022, filed March 21, 1945.

A feature of the invention is a new and improved mirror for a dew point hygrometer.

Another feature is a new and improved optical system employing two mirrors, and a circuit arrangement controlled thereby, whereby the operation of the instrument is made independent of .variations in the intensity of the light source, or

of variations in the amount of light reaching the mirrors due to other causes.

A further feature is a circuit arrangement which utilizes the pulsating output of photocells in an optical system in which the light source is energized by alternating current.

The foregoing and other features of they invention will be described more fully hereinafter, reference being made to the accompanying drawings, in which- Fig. 1 is a vertical section through the mirror chamber and cold box on the line I-I, Fig. 2, showing the optical system and a part of the re,- frigerator;

Fig. 2 is a horizontal section on the line 22, Fig. 1;

Fig. 3 is a partial section on the line 3 3, Fig. l;

Fig. 4 is a sectional View of the dew mirror, on an enlarged scale;

Fig. 5 is a view of the cold rod, in section;

Fig. 6 is a diagrammatic circuit drawing of the complete dew point hygrometer; and

Fig. 7 is a graph showing by means of curves the relation between the amount of dew deposited on the dew mirror and the amount of heat supplied thereto.

Referring tothe drawings, the instrument has a deep rectangular sheet metal base comprising the sides I I and I2 and the rear end I3. The base is, open at the front to give access to the cold box. Extending above the base there is a frame comprising the vertical angle members I4 and I5 in front and the corresponding angle members I5 and I1 at the rear. These angle members may be spot welded to the side members I I and I2 of the basel and are connected by the horizontal members I8 and I9.

The cold box, which is substantially square, is contained in the front part of the base. Above the cold box there is the mirror chamber and the optical system, also the R. F. heating apparatus, as shown clearly in Figs. 1 and 2. The rear portion of the base serves for the mounting of certain items' of electronic equipment, the remainder of which is mounted on a removable chassis adapted to be supported on the horizontal members I8 and I9. This chassis and the rear part of the base are not shown, since the arrangement of the electronic equipment is in accordance with known practice in the radio and allied arts.

The cold box is entirely closed and comprises the sides 20 and 2 I, the back 22, front 23, bottom 24, and top 25. These parts may be of sheet metal except for the top 25 which is preferably made of some good heat insulating material such as Bakelite. The. cold box is supported inside the base by means of screws such as 26 and 21.

The cold rod 28 is located inside the cold box, also the means forextracting heat from the cold rod, said means comprising the evaporator coils 29 and associated parts which are included in the refrigerator.

The refrigerator may be of any suitable type and as shown diagrammatically in Fig. 6 comprises the motor 30, the compressor 3l, pipe 32, condenser 33, liquid reservoir 34, pipe 35, expansion valve 36, evaporator coils 29, and pipe 31 leading back to the compressor. There may also be a vacuum pressure gauge 38 which is connected to the low pressure line 31 by means of a T connector.

The pipe 35 enters the cold box through the rear wall 22, as seen in Fig. 2, and is preferably coiled up inside the heat exchanging cylinder 39, emerging at the other end of the cylinder, where it is connected to a short pipe leading to the expansion valve 35 by the coupling 40. The heat exchange cylinder 39 is not shown in Fig. 6. The circulating system can be further traced by reference to Fig. l from which it can be seen that the outlet from the expansion valve 36 is connected to the evaporator coils 29 by means of a coupling l I, the evaporator coils being connected by means of another coupling 42 to a short pipe 43 extending to the heat exchanging cylinder 39. From the other end of the cylinder the pipe 31, Fig. 2, eX- tends through the rear Wall 22 of the cold box, and leads back to the compressor.

The expansion valve 36 is adjusted by means of a slotted nut 69, Fig. 1, which is ordinarily covered by a cap 62 to prevent the circulation of air around the nut from which frost could form. In order to make this nut accessible from the outside of they cold box an opening 6I is made in the cap 3 62, and an aligned opening 61, Fig. 2, is made in the front wall 23 of the cold box. A short piece of rubber tubing 53 is fitted over the cap 62 and extends forward to the front wall 23. The opening 61 is normally closed by means of a removable cap 66 which is threaded on to a hollow thimble 64 having a ange 65 by means of which it is secured to wall 23 in alignment with opening 51. With this arrangement the valve may be adjusted by means of a screw driver, after removal of the cap 66.

Although omitted from the drawing' to avoid concealing the parts shown, it may be stated that the interior of the cold box should be filled with some suitable fibrous or granular heat insulating material in order to prevent so far as possible the conduction of heat to the evaporator coils and cold rod from the Walls of the cold box. Comminuted cork can be used for this purpose.

Details of the construction of the dew mirror and the cold rod are shown in Figs. 4 and 5.

Referring to Fig. 4, the mirror comprises a hollowl rod 45 made preferably of the alloy known as constantan and has an integrally formed extension 46 of larger diameter which is threaded for connection to the cold rod. The rod 45y has a groove on one side for reception of the copper wire i, which is bonded to a layer 48 of copper deposited on the rod by electro-plating. In the manufacture of the mirror the cylindrical surface of the rod is given a coating 41 of insulating varnish, after which the insulated wire 5i is passed up through the inside of the extension 4E and out through an opening which is in angular alignment with the afore mentioned groove. The bare end of the wire is then secured in the groove by tying it with thread near the point where it protrudes from the insulation. It will be understood that the bare end of wire 5| is insulated from the constantan rod 45 by the layer of varnish 4l'. The rod 45 is then dipped in a colloidal suspension of graphite in alcohol, thinned out by the additionY of carbon tetrachloride, whereby a very thin conductive film is formed on the end of ther rod and on the layer of varnish 4'! whichV covers its cylindrical surface. This film also covers the bare wire 5I. An electrical connection is then made to the extension 45 and to the copper wire 5I and the layer 48 of copper is deposited by electro-plating in known manner.

The end of rod- 45 and the layer of copper depositedv thereon constitute the hot junction of a thermo-couple. The leads from the thermocouple are the wire 5l and a constantan wire 52. which is soldered to the extension 45.

The mirror is finished by plating on a layer of nickel 49 and a very thin layer of rhodium 5!) to form a reflecting surface. This surface is preferably buffed and polished. 'The layer of nickel performs two functions. It facilitates the formation of the reflecting surface of rhodium, which can be deposited more readily on nickel than on copper, and it increases the eniciency of the radio frequency inductive heating apparatus. In further explanation of the increased heating eiiciency it may be pointed out that the induced currents which heat the mirror are largely confined to the outer layer and nickel is a better material for this layer than copper because of its higher resistance.

It should be explained also that prior to the plating on of the nickel layer 49 the portion of the copper layer 43 which is on the end of rod 45 is made extremely thin by lapping and buihng operations. rI'he nickel layer on the end of the rod is also lapped and buffed before deposition of the rhodium layer and should be no thicker than necessary to cover the copper layer. -lill these layers together are very thin, which brings the -thermocouple very close to the reflecting surface of the mirror.

Referring now to Fig. 5, the col-:l rod is preferably made of copper, which is a good heat conductor, and has a helical groove in which the coils 2) of the evaporator are wound. These coils are preferably made of copper tubing and are sweated to the cold rod by solder. The cold rod 28 is hollowl and has a hollow extension 53 at the top which is tapped for reception of the threaded extension 4S of the mirror 45. A lock nut 55 aids in securing the mirror rigidly to the cold rod. The conductors 5| and 52 from the thermo-couple extend down through the hollow cold rod and are brought to the outside of the cold box through a flanged tubular member 68 which is fitted into an opening in the bottom 24. The member GB is made of some heat insulating material such as Bakelite. rihe a1'- rangement described makes it possible to remove the mirror 45 without disturbing the cold rod or other parts in the cold box, which are packed with heat insulating. material' as previously mentioned.

The upper end of the cold rod projects above 'the top 25 of the cold box and is insulated by means of the ilanged cylindrical member 55 and the washer 55, which define an annular space around the extensionl 53 of the cold rod. The member 55 and the washer 56 are made of insulating material such as Bakelite or polystyrene and the space around the extension 53 is lled with insulating material. The cylindrical member 55 is surrounded by a metalsleeve 58 for mechanical protection and its lower flanged end rests on arcork gasket 59 which is seated on the col-d rod 28. The washer 56 rests on the top of member 55 and its periphery extends into an annular recess formed in the metal sleeve 58. These parts are clamped together by means of a polystyrene nut 51 which is threaded on to the extension 45 of the mirror @5. The Bakelite top 25 of the cold box has an opening through which sleeve 55 projects and the top engages the flange on the cylindrical member 55, as shown in Fig. 1.

The washer 55 has a slotted raised sector 19, to which there is secured a small block 'H of thermal insulating material. This block supports the dummy mirror 12, which may be a short piece of brass rod plated with rhodium on the end to form a reflecting surface similar to the reecting surface on the mirror 45.

The mirror chamber is formed in part by the bottom member 11 which rests on the top 25 of the cold box and has an opening for receiving the sleeve 58. Supported on the bottom 11 there is a one piece casting comprising the top 13, bored out to receive parts of the optical system, the ends 14 and 15, and the rear wall 15. The front of the mirror chamber is closed by the front panel 18, which has a window 19 through which the mirrors may be inspected during operation of the machine. This window also gives access to the mirrors whenk they have to be cleaned.

The pipes 8l and 32 are threaded into the end walls 14 and 15, respectively, of the mirror chamber. These pipes are included in the system for circulating the air or other gas, the'dew point of which is to be determined, through the mirrorV chamber. In case the dew point of the outside air is to be determined, for example, the circulating system may include a blower (not shown) having an intake pipe extending outside the building in which the instrument is located and an exhaust pipe connected to the pipe 82, whereby a stream of air may be directed through the mirror chamber. No connection is made to pipe 8|, unless the dew point of some gas other than atmospheric air is to be determined, in which case it may be necessary to provide a closed circulating system.

The plate 83 on which the lamp 84 and photocell 85 are mounted is fastened to the vertical members |4 and |5 of the frame by means of screws, as seen in Fig. 1, and is also secured to the rear wall 16 of the mirror chamber. There is an opening 80 in this wall and a corresponding opening in plate 83 which are covered by a plate 81 of insulating material. The screws 88 and 89, Fig. 2, pass through plate 89 and plate 83 and are threaded into wall 16.

The terminals 90 and 9| are mounted on the insulating plate 81 and extend into the mirror chamber where they support the radio frequency heating coil 92. This coil, consisting of about 4 or 5 turns, surrounds the dew mirror 45. Outside the mirror chamber the terminals 90 and 9| are connected to and support the one turn secondary winding 93 of an air core transformer. The primary winding 96 of this transformer, shown in Fig. 6, is wound on an insulating tube 94. The grid coil 91 is wound on a tube 95 of smaller diameter and both tubes are clamped to a plate 98 of insulating material by means of the cross member 99 and the brass. screw |00. The plate 98 may be secured to the top 25 of the cold box by screws and has channels for the conductors leading to the coils 93 and 91.

The light source 84 may be an automobile headlight lamp of the prefocused type having a socket which is secured to the plate 83 by a clamp |02. The lamp socket should be so oriented that the axis of the lamp filament is perpendicular to the plate 83. The reference character |03 indicates a condenser lens. This lens is carried by a conventional lens mount |05 the lower end of which is fitted to one of the bores in the top 13 of the mirror chamber. The focus of the lens is some distance beyond the mirrors 45 and 12 so that the reflecting surfaces of both mirrors are covered by the light projected from lamp 84.

The reference character |04 indicates an infra red filter which is provided to prevent heat rays from reaching the mirrors.

The photocell 85 may be a Cetron type CE-Z l-D gas filled twin phototube and has cathodes |01 and |09 and anodes |08 and ||0, as seen in Fig. 3. The tube is enclosed in a suitable casing of which the bottom and one end are formed by an L- shaped piece |2 Which is attached to the bracket H3 by means of two screws which also serve to fasten the socket I4 in the openingr provided for it. The bracket ||3 is attached to the plate 83 by screws as shown. The casing is completed by a cover H5 of U -shaped cross section which slides on to the bottom The light beams reflected from the mirrors 45 and 12 are projected on to the cathodes |01 and |09, respectively, by the lens ||8 through the openings Elli and ||1 in the bottom of the casing which encloses the phototube 85. IThe lens H8v is a plano convex lens and is supported in a bore in the top 13 of the mirror chamber conventional manner. The reference character I |9 indicates alight spreading disc. Between the lens ||8 and disc I|9 there is a mask |20, which may be formed by a strip of opaque tape or other material. This mask insures that the light reaching each cathode is confined to that which is reflected by the associated mirror, and prevents cathode |01, for example, from receiving any` light which is reflected from the mirror 12.

Secured to the bottom of the casing for' the phototube there is a block |2| which is drilled and tapped for the two screws |22 and |23. These screws are located in front of the openings IIE and and may be turned in or out to block these openings more or less against the passage of light. The purpose of this arrangement will be explained presently.

Referring now to Fig. 6, the conductors 5| and 52 from the thermo-couple in the mirror 45 extend to a graphic recorder |25, which may be of any suitable type, such as the Micromax recording potentiometer manufactured by Leeds and Northrup Company, for example. This instrument includes a temperature compensated cold junction and may be calibrated in micro- Volts or in degrees Fahrenheit or centigrade as desired. If the instrument is calibrated in micro-volts the dew point temperature corresponding to any reading is found by reference to a table showing the temperaures which correspond to various voltages.

The optical system comprising the light source 8d, the mirrors 45 and 12 and the phototube 85 is shown diagrammatically to the right of the recorder |25.

The circuits which include the phototube 85, the tube |30 and the balancing transformer |40 are designed to balance out variations in the intensity of the light source 84 and to derive an output which is proportionate to the amount of dew on the mirror 45. The tube |30 may be a type 6SN7 twin triode.

The tubes |3| and |32 and associated circuits constitute a two stage amplifier for amplifying the output of the transformer |40. The tube |3| may be a type GSK? variable me pentode, While tube |32 is preferably a type 6V 6 beam power amplifier.

The tubes |33, which may be a type BSH? pentode, and |34, a type BHS diode rectifier, are included in an automatic volume control circuit the purpose of which will be explained. The circuit includes the potentiometer |4| from which a negative grid bias is supplied to tube |3| and by means of which the amount of grid bias can be adjusted.

The radio frequency oscillator includes the primary winding or tank coil 95, the grid coil 91 and the tube |38, which may be a type 6L6 beam power tube. An ammeter is indicated at |42. The ammeter and a relay |43 are in the cathode plate circuit of the oscillator tube and the relay is arranged to shut d-own the refrigerator when the oscillator output reaches a predetermined value. The frequency of the oscillator may be on the order of 500,000 cycles per second.

The output of the oscillator is controlled by the voltage developed across the resistor |44-which is included in the cathode plate circuit of tube |33. This voltage is applied to the screen grid of tube |33 through contacts of the defrost switch S3. This switch-makes it possible to apply the full +B potential to the screen of the tube |38 to increase its output to the maximum value and is employed when it is desired to defrost or clear the dew from the mirror 45.

The tube |36 may be a type 6F5 Hi mit triode. The current iiow in this tube determines the amount of voltage developed across the resistor |44 and is dependent upon the difference between the voltages developed across the potentiometers |45 and |45, which are connected in the cathode grid circuit of the tube in series opposition. The voltage across potentiometer |46 is developed by a current produced by rectification of commercial alternating current through the medium of the transformer |41 and the diode rectifier tube |35, while the voltage across the potentiometer |45 is developed by current produced by rectification of the signal output of tube |32 by means of transformer |48 and the diode rectifier tube |31. The potentiometer |46 is used to adjust the threshold of control, that is, to x the lower limit of the range of dew Values used for control purposes. Potentiometer |45 is used to adjust the sensitivity or rapidity of the response to a change in dew value. All this will be more fully explained in the course of the explanation of the operation of the hygrometer.

Direct current for the plate circuits of the various tubes is supplied by a well known type of full wave rectifier which includes the rectifier tube |39 and transformer |49. The conductors marked vr-- supply current at low voltage to the heater circuits of the tubes (not shown) and also supply current to the lamp 84.

The switch S2 is the main switch through which commercial alternating current is supplied to transformers |41 and |40. The switch Si supplies current to the motor 30 which drives the compressor 3| and makes it possible to start er sto-p the refrigerator independently.

The operation of the dew point hygrorneter may now be explained. It will be assumed for the purpose of the explanation that the hygrometer is to be used for determining and recording the dew point temperature of the air at some location where the information is required, such as a station of the Weather Bureau, for example. A blower and hose connection will therefore be arranged to supply a stream of air from outside the building to the pipe 8| or 82 of the hygrometer, as previously mentioned.

The hygrometer is started by closing the switches Si and S2. The cord (not shown) for supplying operating current to the recorder |25 should also be connected to some convenient outlet. lThe chart |50 will now begin to move downward and the stylus will begin to draw a line thereon indicative of the temperature of the mirror 45. The temperature at the start will of course be the ambient temperature of the air supplied to the mirror chamber.

The closure of switch Sl starts the motor 30 and the compressor 3|. In a short time refrigerant will be supplied to the evaporator coils 29 and the extraction of heat from the cold rod 28 and mirror 45 will begin. The temperature of the mirror will accordingly start to go down.

The closure of switch S2 starts the rectifier and the various tubes in the control circuit are rendered operative. The lamp 84 is also energized. The light from the lamp 84 is projected on to the mirrors 45 and 12 whence it is reected to the photocell 85, the light beam from mirror 45 impinging on cathode |0'| and the beam from mirror 12 impinging on the cathode |09.

A flow of current is accordingly established in la circuit which may be traced from ground by way of cathode |01, anode |08, resistor |53, upper half of potentiometer |52 and resistor |55 to the junction point |56 onV the voltage divider 51- |58, which is providedbecause the phototube requires a somewhat lower anode potential than the other tubes. Current ow is also established in a similar circuit which includes the cathode |09, anode l0, resistor |54, and the lower half of potentiometer 52.

The voltages which appear on the anodes |08 and ||0 due to current flow in the above described circuits have small cycle pulsating components which are caused by uctuations in the amount of light emitted by the lamp 84. The lamp is energized by 60 cycle alternating current and although the light emitted may appear steady to the eye it nevertheless diminishes slightly each time the energizing current falls to zero, or ateach half cycle. The frequency of the light fluctuations is therefore double the frequency of the energizing current.

The pulsating voltages produced at anodes |08 and ||0 as above explainedfare applied by way of condensers |59 and |60 to the grids of the tube |30 where they produce amplified current fiuctuations in the plate circuits of the tube. These circuits include, respectively, the upper and lower sections of the primary winding of the balancing transformer |40, and since the currents flow in opposite directions in these sections they oppose each other. If the currents are equal and in phase the result is zero output from the secondary winding.

The currents will be in phase or substantially so but they will not ordinarily be equal unless the instrument has previously been adjusted. It will be assumed, therefore, that a, condition of imbalance exists, producing 120 cycle alternating voltages at the secondary winding of the transformer |40 which are applied to the control grid of the tube |3|. This tube operates as an amplier and amplied voltages are transmitted by way of condenser |6| to the control grid of tube |32, where they are further amplified. The plate circuit of the tube |32 includes the primary Winding of the transformer |40, and the fluctuating currents in the plate circuit accordingly generate alternating voltages across the secondary winding of the transformer. Current flow is thus established alternately in two circuits which include, respectively, the upper and lower sections of the double diode rectifier tube S'Iand the upper and lower sections of the secondary winding of the transformer. The current flow in the potentiometer |45, which is common to both circuits, is unidirectional and produces a unidirectional voltage across the potentiometer. The voltage peaks are smoothed out by the condenser |62, so that a fairly constant potential is produced the value of which depends on the degree of unbalance at the transformer |40.

It may be noted now that alternating current is supplied to the primary winding of the transformer |4'i through the switch S2, producing alternating voltages across the secondary winding, whereby a unidirectional current ow is established in the potentiometer |46 by means of the double diode rectifier tube |35 and the circuit hook-up shown. The voltage thus developed across the potentiometer is made fairly constant by the condenser |63.

The oscillator control tube |36 has a cathode grid circuit which may be traced from the cathode by way of part of the winding of potentiometer |46, the slider |65, part of the winding of the potentiometer |45, and the slider |64 to the grid. The voltage developed across the potentiometer winding |46 has such a polarity (indicated in the drawing) that it tends to produce a negative bias on the grid while the voltage developed across the potentiometer |45 has the opposite polarity and tends to produce a positive bias on the grid. The effective grid potential with respect to the cathode is therefore equal to the difference between these two potentials.

The adjustment of the potentiometers |45 and |46 will be discussed later. However, it may be stated at this point that the potentiometer |46 should be so adjusted that with no voltage across potentiometer |45 the tube |36 is biased below cut off and passes no current. The voltage across the part of the potentiometer |46 which is in the grid circuit may be about 35 volts, for eX- ample.

It may be assumed that the amount of unbalance at transformer |40 is considerable, with the result that sufficient potential is developed across the potentiometer |45, or rather that part of it 4which is included in the grid circuit of tube |36,

to reduce the negative potential on the grid far enough so that current can flow in the cathode plate circuit of the tube. The positive voltage, which is in opposition to the negative voltage derived from potentiometer |46, is indicated on the voltmeter |66 and may be 25 volts or more. The cathode plate circuit of the tube |36 includes the resistor |44. Current flow in the circuit causes a positive voltage to be developed on conductor |61 which is applied to the screen grid of the oscillator tube |38 by way of the normally closed contacts of the defrost switch S3. The resistor |44 may have a value of about 100,000 ohms, for example, which is suicient to give a good range of voltages on conductor |61 responsive to changes in the current flow through the control tube |36.

At times when the tube |36 is not passing current, that is, when it is biased below cut off, the conductor |61 and the screen grid of the oscillator tube |38 are at ground potential and the oscillator is inoperative. When a positive potential appears on the screen grid, due to current flow through the control tube |36, the oscillator becomes operative and high frequency currents are generated in a circuit which includes the secondary winding or loop 93 and the heating coil 92. High frequency currents are accordingly induced in the mirror 45 which because of the skin effect are confined mainly to the nickel layer 49. Nickel is a material which is readily heated by induction, and since the underlying layer of copper 48 is a good heat conductor the heating arrangement is relatively eiicient.

The reading of the ammeter |42 is indicative of the rate at which heatis supplied to the mirror. The actual temperature of the mirror is indicated by the recorder |25. If there should be danger of overheating, due to an excessive degree of unbalance at transformer |40, the rate can be reduced quickly by adjustment of the potentiometer |45.

The foregoing describes the conditions which prevail immediately after the liygrometer is started up. Reviewing the situation briefly, the refrigerator has been started and is working to extract heat from the cold rod 26. The optical system is in operation, but due to unequal outputs from the twosections of the phototube, a condition of unbalance exists at the transformer |40, producing enough signal output from the transformer to start the radio frequency oscillator.

10 Heat is accordingly being 'supplied to the mirror, and its temperature is indicated on the recorder |25.

The unbalanced condition may be due to one or more of several causes. For example, the two mirrors 45 and 12 may not have exactly the same reflecting power, or the two sections of the phototube may not give equal outputs for equal amounts of light received, or the two sections of tube |36 may not have the same gain. These and other causes can be expected to be present at the start in greater or lesser degree and the instrument will ordinarily be out of balance, therefore, as has been assumed to be the case.

In order to adjust the instrument to a balanced condition, the operator Will first make sure that the slider of the potentiometer |52 is adjusted to mid-position and will see to it that the adjusting screws |22 and |23 are backed off far enough so that they are entirely out of the paths of the light beams with which they are associated. The operator will then turn in one of the adjusting screws, screw |22 for example, far enough to cause the end of the screw to enter the path of the associated light beam, and will note the effect on the voltmeter |66. If the voltmeter shows an increased reading the operator will be advised thereby that the circuit is unbalanced to a greater extent than it was before and he will restore screw |22 to its original position. Screw |23 is now turned in until it begins to intercept the associated light beam, with the result that the voltmeter |66 will show a decreased reading, indicating that the unbalanced condition has been improved. The adjustment is then continued until the voltmeter reads zero, or as near to zero as possible. There may be a slight phase displacement between the currents in the two sections of the primary winding of transformer |40 which will make it impossible to obtain an absolute balance, but an approximate balance can easily be obtained.

it will be understood that while the adjustment described in the foregoing is being made the temperature of mirror 45 is above the dew point and both mirrors are clear of dew.

The phototube circuits having been balanced, the output of transformer |40 is reduced to zero, or substantially Zero, and the voltage developed across the potentiometer |45 is zero, or substantially zero. The voltage developed across 'the potentiometer |46 is now unopposed and effectively biases the tube to cut oif. Current flow through the tube ceases, the potential on the screen grid of the radio frequency oscillator is reduced to ground potential and the oscillator stops oscillating, with the result that the supply of heat to the mirror is shut off.

As the refrigerator continues to operate the temperature of the mirror 45 will fall and will eventually reach the dew point of the air circulating through the mirror chamber. Dew now begins to form on mirror 45, which diminishes its reflecting power. The temperature of mirror 12 remains substantially the same as thatk of the circulating air, since it is insulated from the c old rod, no dew forms on this mirror, and its reflecting power remains unimpaired. As dew forms on mirror 45, therefore, the output of the lill-408 section of photctube S5 decreases, whilejtheoutput of the other section remains the same, thereby `again creating a'condition of unbalance at the transformer |46.

former i, which is amplified and rectified as described and produces a potential across the potentiometer |45. This potential increases as the amount of dew increases, that is, the potential across the potentiometer is proportionate to the amount of dew on mirror 45, and very soon becomes large enough to cause the control tube |353` to again become conductive. The potential on conductor |61 accordingly again rises above ground potential, likewise the potential on the screen grid of the oscillator tube |33, and the oscillator again begins to function to supply heat to the mirror 45.

The oscillator output at this stage is low and only enough heat is supplied to the mirror 45 to compensate for the heat withdrawn by the cold rod and maintain the temperature of the mirror constant, or very nearly so. The control circuits are extremely sensitive, however, and very slight changes in the amount of dew deposited on the mirror are capable of enormously increasing the oscillator output. Thus as the temperature of the cold rod goes down, heat will be extracted from the mirror at a higher rate, its temperature will fall very slightly and the amount of dew on the mirror will be minutely increased, thereby increasing the output of the oscillator enough to make up for the higher rate of heat loss. The temperature of the cold rod will eventually reach the limit imposed by the nature of the refrigerant being used, as low as 60 F., for example, and the amount of heat supplied to the mirror will become stabilized, except for changes in the dew point. If the dew point is fairly high, |50 F., for example, a considerable amount of heat will have to be supplied to maintain the mirror at this temperature. The amount of dew on the mirror, however, has increased by only a very small amount.

The line drawn on the chart at the recorder now shows the dew point temperature and will continue to do so, except when the mirror is defrosted during occasional balance tests, as will be mentioned hereinafter.

The relation between the amount of dew on mirror 45 and the heat output to the mirror may be explained with reference to Fig. '7, where it is depicted by the curve |16. Three ranges of dew are taken into consideration, an invisible range |11, in which the dew on the mirror is not visible to the eye, a so called useful range |12, in which the dew is visible and is used for the purpose of control, and an indenite range |13 in which the dew is heavier and is not used for control purposes. The temperature change over the whole range of dew values shown is quite small. Over the useful range it is only a small fraction of a degree.

The curve is very steep and indicates that the heat output to the mirror increases very rapidly with an increase in the amount of dew within the useful range. This range of dew values is suicient to control the oscillator over its whole working range extending from substantially zero output to maximum output.

The location of the curve within the useful range of dew values is determined by the adjustment of the potentiometer |46, which determines the amount of negative bias on the grid of the control tube |36. The phototube 85 is extremely sensitive and can detect the formation of dew before it becomes visible and it is possible therefore to use a range of dew values which is partly or wholly within the invisible range for the control of the hygrometer. Curve. |14 illustrates this operating condition. The dew point, however, is by accepted definition the temperature at which a visible deposit of dew is formed and it follows that if the hygrometer were to be operated in accordance with curve |14 it would indicate too low a temperature for the dew point. It is necessary, therefore, to utilize for control purposes a range of dew point values over which the dew on the mirror is visible. This range may be selected at a time when the oscillator output is very low. as indicated by the ammeter |42, by observing the mirror 415 and adjusting the potentiometer |45 until the deposit of dew on the mirror can just barely be seen with the eye.

The slope of the curve |10 is determined by the sensitivity yof the control circuits, or the rapidity of the response to slight changes in the amount of dew, and may be adjusted by means of the ypotentiometer |45. The sensitivity should not be too low, a Icondition illustrated by the curve |15 which extends outside the useful range of dew values. On the other hand it is possible to make the sensitivity too high, in which case the instrument is apt to manifest a, tendency to hunt. A condition of hunting is indicated by the periodic 'appearance and disappearance of the dew, also by a periodic change in the reading of the ammeter |42 and the voltmeter |66. In practice the sensitivity is adjusted to be as high as possible while maintaining a continuous deposit of dew on the mirror.

`The double mirror system tends to compensate for changes in the intensity of the light source 84, such as may be due to changes in line voltage. When the machine is in operation there is a certain amount of dew on the mirror 45, which is being maintained by the simultaneous flow of heat to the cold rod and the supply of heat by the oscillator. Now equal amounts of light reach the two mirrors and if the line voltage falls the decrease in the amount of light reaching mirror 45, which would otherwise be interpreted as an increase in the amount of dew, is compensated for by an equal decrease in the amount of light reaching mirror 12.

As regards the reected light the situation is different since the instrument operates in accordance with the difference between the amount of light reflected by mirror 45 and the amount of light reflected by mirror 12 and this difference changes percentagewise with changes in voltage `which affect the intensity of light source 8|. If the voltage should fall, therefore, the output of transformer |40 will fall even though the amount of dew on mirror 45 remains constant. This reduces the sensitivity of the control circuit, which is also reduced at other points, due to change in the gain of the amplifiers, for example.

The automatic volume control circuit is employed in order to maintain the sensitivity of the control circuit at a high level notwithstanding such factors as a drop in line voltage and accumulation of dust in the optical system which tend to reduce it. Explaining this further, it will be noted that the pulsating voltages produced at the anode l I0 of the phototube are not only impressed on the grid at one section of tube |30 but are also impressed Aon the grid of tube |33, these grids being connected by a conductor |12. The output of tube |33 is rectified by the arrangement including transformer |13 and the diode rectifier tube |34 to cause a flow of direct current in a circuit which includes the winding of the potentiometer |4|, thereby providing a negative grid bias for the amplifier tube |3l. A

potentiometer is Iused rather than a fixed resistor so that the best value of negative bias can be selected depending on the gain of tube |33, the sensitivity of the .phototube and other factors.

If we assume now that the hygrometer is operating with the control circuit in a desired condition of sensitivity, as determined by the adjustment of the potentiometer |45, 4a fall in line voltage will reduce the amount of light reflected from mirror '|2, thereby reducing the output of tube |33 and reducing the negative grid bias at tu-be |3|. The gain of tube |3| is thus increased land the sensitivity is maintained at its former level. It will be seen therefore that the potentiometer |45, having once been adjusted, will require a minimum of attention, since the volume control circuit automatically takes care of changes which would ordinarily require changes in the adjustment of the potentiometer.

If the air -circulating through the mirror chambei' is not entirely clean there will be a certain amount of dirt or dust deposited on the reflecting surfaces of the mirrors which gradually impairs their reflecting powers. It also tends to unbalance the phototube circuits since the mirror 45, bemg wet with dew, tends to accumulate dirt faster than the mirror '|2. The instrument can be tested occasionally for an unbalanced condition by operating the defrost switch S3. When this switch is operated the full -i-B voltage is mpressed on the screen grid of the oscillator tube |38 and the oscillator `output is increased to the maximu supplying heat enough to the mirror so that the dew or frost will quickly disappear. The operator will then note if the reading of the voltmeter |66 has dropp'ed to Zero, or substantially zero, and if it has not he will correct the unbalanced condition by adjusting the potentiometer |52. The provision of the potentiometer Iavoids the necessity -of cleaning the mirrors too frequently. Under ordinary conditions this need not be done oftener than about once every twenty-four hours.

The relay |43 is included in the cathode plate ci-rcuit of the oscillator tube |33 and is adjusted to operate when the current flow reaches a predetermined value. The function of the relay is to shut down the refrigerator periodically at times when its -continuous operation is not required, thereby power required to run the refrigerator `and a similar reduction in the amount of power expended at the oscillator. Continuous operation of the refrigerator is unnecessary at times when the -dew point is relatively high, 40 F. or higher, for example, since at such high dew point temperatures it is unnecessary to maintain the cold rod at an extremely low temperature and to do so involves a waste of power both at the refrigerator and the oscillator.

Describing the operationbriefly, it may be assumed that the dew point has been low and has just gone up high enough so that the power consumed at the oscillator is sufficient to operate relay |43. When the relay energizes, it opens the circuit of motor 39, thus stopping the motor an-d the compressor 3|. The temperature of the cold rod 28 now rises gradually, heat is `extracted from the mirror at a progressively decreasing rate, and the power consumption at the oscillator decreases in corresponding manner. After a time the current in the cathode plate circuit of oscillator tube |38 will have decreased enough to permit the relay |43 to fall back and again close the circuit of the motor 30. The refrigerator is thus started `effecting a reduction in the amount of up again 'and when the temperature of the cold rod has been reduced to its former value the relay |43 again energizes to repeat the cycle, assuming that the dew point has remained at a high enough temperature.

The invention having been described, that which is believed to be new and for which the protection of Letters Patent is desired will l'be pointed out in the appended claims.

I claim:

1. In -a mirror for a dew point hygrometer, a metallic body one end of which is adapted to be connected to a cold source, a cup shaped inner metal shell enclosing the other end of said body and joined to the body lat the end surface thereof to form the hot junction of a thermo-couple, an outer metallic shell enclosing said inner shell and joined to the outer surface thereof, and -a mirror surface formed on the end of said outer shell. f

2. In a mirror for a dew point hygrometer, a met-allie rod forming one element of a thermocouple, Aa thin coating of a different metal deposited lon said rod and f-orming the other element of said thermo-couple, said coating being insulated from the side wall of said rod and in conductive contact with said rod at the end thereof, and means comprising another metallic coating deposited on the end of said rod outside said rst mentioned coating to form a `reflecting surface.

3. In a mirror for a dew point hygrometer, a metallic rod forming one element of a thermocouple, a thin coating of a different metal deposited on said rod and forming the other element of said thermo-couple, said coating being insulated from the side wall of said rod and in conductive contact with said rod at the end thereof, a second coating coextensive with said first coating and in conductive contact therewith, said second coating being formed of a metal having a higher resistance than the metal of said first coating, and a thirdv coating deposited on said second coating and forming a reflecting surface at the end of the rod.

4. In a mirror for a dew point hygrometer, a rigid metallic body forming one element of a thermo-couple and adapted to function as a heat conductor, a metallic layer having a limited area thereof in conductive contact with said body and forming the other element of said thermo-couple, means forming a reflecting surface on said layer over an area which is coextensive with the area in contact with said body, and a second metallic layer in conductive contact with an area of said rst layer outside said reflecting surface, said second layer being made of a metal adapted to be heated efficiently by high frequency induction.

5. In a dew point hygrometer, a passage for the flow of gas, first and second mirrors located close together in said passage, means for projecting a beam of light impinging on both said mirrors, means for cooling the first mirror to the dew point of said gas, whereby the reflecting power of the first mirror is reduced by dew deposited thereon, means for heating said first mirror, and means for regulating said heating means in accordance with the difference in the amount of light reflected by said mirrors to maintain said first mirror at the dew point.

6. In a dew point hygrometer, an element having a light reflecting surface, means for cooling said surface to the dew point in the presence of a gas the dew point of which is to be deter- Vmined;whereby the reflecting power of said sur- *facet is 'reduced bygthedeposit of dew thereon, a

means, and means sensitive to the dierencebetween the amounts of light refiected by said surfacesto maintain the reflecting surface of said first element at the dew point by regulation of said heating means.

7. In a dew point hygrometer, a mirror on 4which dew is deposited at the' dew point temperature, means for uniformly cooling the refleeting surface of said mirror, means for heating said mirror, -a second mirror maintained above the dew point temperature, two photocells, an optical system including a source of light and ysaid mirrors for projecting two light beams to said photocells, respectively, before the deposit of dew on said first mirror begins, a control circuit Vfor regulating said heating means, means including an amplifier for producing a voltage in said control circuit having a value which depends on the difference between the outputs of said photocells after Vdew has been deposited on said rst mirror, and means for maintaining a constant ratio between said voltage and the amount of dew on said first mirror notwithstanding changes in the voltage of the power supply to said amplifier, said last means comprising an automatic volume control circuit in which a negative potential is developedproportionate to the outputof one of said photocells, and a connection over which a portion of said negative potential is applied to said amplifier to regulate the output thereof.

8. In a dew point hygrometer, a mirror on which dew is deposited at the dew point temperature, a source of light, means for projecting a beam of light from said source to said mirror, a second mirror maintained above said temperature and located adjacent said first mirror, to intercept a part of said beam, two photocells to which light beams are reflected by said mirrors, respectively, and means for throttling either of said last mentioned light beams at will to equalr ize the output of said photocells at a time when there is no dew on said first mirror.

9. In a dew point hygrometer, a dew mirror and a dummy mirror, two photocells, an optical system including a source of light and said mirrors for forming two light .beams and projecting them on to said photocells, respectiveiy, means for amplifying the outputs of said photocells comprising two space discharge devices having their control grids connected to the anodes of said photocells, respectively, a transformer, and plate circuits for said devices including, respectively, two opposed sections of the primary winding of said transformer, whereby the current in the secondary winding of said transformer is proportionate to the difference between the outputs of saidphotocells.

10. In a dew point hygr'ometer, an element having a light reflecting surface, a refrigerating system for cooling said element, means including a high frequency oscillator for generating heat in said element by induction, said oscillator including a space discharge device, means for controlling the output of said oscillator to maintain the temperature of said element at the dew point,

I6 and: means includingV a relay sensitive to current owin the `anode circuitcf said device for stopping said refrigerating system responsive to the oscillator output reaching a predetermined value and for starting it responsive to said output reaching a predetermined lower value.

11, In a dew point hygrometer, an element having a light reecting surface, means for cooling said element to cause the deposit of dew on said surface, a second element adjacent said first element and having a light reiiecting surface, means for projecting a light beam on to said surfaces, two photocells to which light beams are reflected by said surfaces, respectively, at times when the first surface is free of dew, the deposit of dew on said first surface being effective to diminish the intensity of the beam refiected from such surface to the associated photocell, circuits forsaid photocells including a source of current, means responsive to current flow in said circuits for generating an alternating current which is proportional to the difference between the-intensities of the beams reflected by said surfaces, means for rectifying said alternating current, a circuit including a source of direct current, means for deriving a control voltage which is proportionate to the difference between the rectified alternating current and the current in said circuit, and means for heating said element regulated by said control voltage to maintain the said first reflecting surface at the dew point.

l2. In a dew point hygrometer, an element having a light reflecting surface, means for cooling said element to cause dew to be deposited on said surface, means including an oscillator for heating said surface, said oscillator including a space discharge device having a screen grid, a second space discharge device constituting a control tube, a source of anode current for said devices, a resistor connected between the cathode of said control tube and the negative terminal of said source, a conductor extending from said screen grid to the junction of said resistor and said cathode, whereby said oscillator is rendered inoperative by the negative potential at said junction when no anode current is flowing in said control tube, means responsive to a reduction in the light reflected from said surface by deposit of dew thereon for causing anode current to flow in said control tube to thereby raise the potential at said junction and start said oscillator.

13. A dew point hygrometer as claimed in claim 12, wherein aswitch is provided in said conductor to disconnect the said screen grid from said junction and connect it to the negative terminal of said source, to thereby stop said oscillator when the current is flowing in said control tube.

14. In a dew point hygrometer, a cold rod made of good heat conducting material, a metallic member secured in a recess at one end of said cold rod and constituting one element of a thermocouple, said member having supported thereon in superimposed relation a metallic layer constituting the other element of said thermocouple and a coextensive metallic layer having a reflecting surface, a channel extending through said cold rod and said metallic member, a conductor extending through said channel and having a connection to said first metallic layer, and a second conductor connected to said metallic member and extending through that portion of said channel which is in said cold rod.

15. In a dew point hygrometer, a cold rod of good heat conducting material and having a longitudinal channel extending therethrough, an extension of said cold rod secured thereto at one end, said extension having a channel therein communicating with the channel in said cold rod and being closed at the other end, a thin layer of metal covering the end of said extension, said layer and said extension being made of different metals and constituting the hot junction of a thermocouple, a metallic layer covering said first layer and having a refiecting surface, an opening in the side of said extension communicating with the channel therein, and two conductors extending through the channel in said cold rod, one of said conductors being connected to said extension and the other conductor extending through the said channel and opening in said extension and having a connection to said rst layer.

16. In a dew point hygrometer, an element having a light reflecting surface the reiiecting power of which is diminished by deposit of dew thereon, means for uniformly cooling said surface to the dew point to start the deposit of dew thereon, means for uniformly heating said surface, a second element having a light reflecting surface, means for projecting light on to said surfaces, two photocells to which light beams are reflected by said surfaces, respectively, before the deposit ofv dew begins, and means responsive to the deposit of dew on said first surface and the resulting excess in the amount of light reaching the second photocell from the second surface for starting the operation of said heating means.

17. In a dew point hygrometer, a mirror on which dew is deposited at the dew point ternperature, means for cooling said mirror, a second mirror adjacent to said first mirror but insulated from said cooling means, a source of light, means including said source and a lens 'for projecting a beam of light impinging on both said mirrors, two photocells in close proximity to each other, means including another lens for projecting the beams reflected from said mirrors to said photocells, respectively, and means for adjusting the cross section of at least one refiected beam to equalize the amounts of light reaching said photocells at a time when said first mirror is free of dew.

18. In a dew point hygrometer, a mirror, means for cooling said mirror to dew point temperature, a second mirror maintained substantially at ambient temperature, a periodically varying light source arranged to project light on to said mirrors, two photocells to which said mirrors reflect light beams, respectively, two circuits for said photocells, respectively, in which pulsating voltages are generated in response to periodic variations in the intensity of said light beams, two amplifiers, condensers coupling said photocell circuits to said amplifiers, respectively, whereby said pulsating voltages are transmitted to the amplifiers and are amplified thereby, means for comparing the outputs of said amplifiers, means supplying heat to said first mirror, and means for regulating said heat supplying means in accordance with the difference between said amplifier outputs to maintain said first mirror at the dew point.

19. In a dew point hygrometer, a dew mirror and a dummy mirror, two photocells, a periodically varying light source, means including said source and said mirrors for forming two light beams and for projecting -them on to said photocells, respectively, circuits for said photocells including a source of direct current and in which potentials are developed at one terminal of each of said photocells in response to the impingement of said light beams thereon, said potentials each having a pulsating component produced by the variations in the intensity of said light beams, means for equalizing said pulsating components when said dew mirror is free of dew, two amplifiers arranged to amplify said pulsating components, respectively, and means for comparing the outputs of said amplifiers and for deriving an alternating current proportionate to the difference between said components when said dew mirror has dew thereon.

20. A dew point hygrometer as claimed in claim 19, wherein the comparing means comprises a transformer having two primary windings energized by the outputs of said amplifiers, respectively, and a secondary winding in which the said alternating current is generated.

21. In a dew point hygrometer, a mirror on which dew is deposited at the dew point temperature, means for cooling said mirror, means for heating said mirror, a circuit including the resistance element of a potentiometer, means responsive to the deposit of dew on said mirror for producing direct current flow in said circuit proportionate to the amount of dew, a second circuit including a source of direct current and the resistance element of a second potentiometer, means for regulating said heating means in accordance with the current flow in said first circuit, said regulating means including a space discharge device, and a cathode grid circuit for said device including the sliders of said potentiometers and portions of the resistance elements thereof all connected in series, the slider of the first potentiometer being connected to the grid of said device, and the direction of current fiow in said circuits being such that the voltages across the included portions of the resistance elements of the said potentiometers are in opposition and produce a positive potential at the grid of said device only if the voltage across the included portion of the resistance element of the first potentiometer is the greater of the two voltages.

22. In a dew point hygrometer, a mirror on which dew is deposited at the dew point temperature, means for cooling said mirror, means including an oscillator for heating said mirror by induction, a photocell to which a beam of light is reflected by said mirror when no dew is deposited thereon, means including said photocell for generating an alternating current the value of which is inversely proportionate to the amount of light in said beam, an amplifier and means for transmitting said alternating current thereto, means for rectifying the output of said amplifier to produce a control voltage, means responsive to said control voltage for starting said oscillator, means for generating a voltage in opposition to said control voltage, and means for adjusting said opposing voltage to a value which reduces the control voltage sufficiently to prevent the starting of said oscillator until a predetermined amount of dew has been deposited on said mirror.

DE WITT T. VAN ALEN.

(References on following page) REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PA'IENTS Number Name Date George Sept. 6, 1938 Shepard Dec. 6, 1938 Guiilksen Dec. 13, 1938 Thornthwaite Apr. 29, 1941 Thornthwate Jan. 6, 1942 Number 2G Name Date Bierwirth Sept. 1, 1942 Turin May 15, 1945 Graves et al Aug. 7, 1945 Walton Feb. 11, 1947 Brown May 20, 1947 Frswold et al Apr. 12, 1949 OTHER REFERENCES w Publication, Journal of Scientific Instruments,

vol. 14, 1937, pages 811-88. 

